Bonding with vinyl acetate polymer modified resorcinol-base adhesive

ABSTRACT

THIS INVENTION RELATES TO A METHOD OF GLUING WOOD, WOOD ARTICLES AND THE LIKE WITH RESORCINOL-BASED RESIN ADHESIVES, SAID ADHESIVES COMPRISING A RESORCINOL-BASED RESIN AND A VINYL ACETATE POLYMER IN AN AMOUNT UP TO ABOUT 10% OF THE RESORCINOL-BASED RESIN SOLIDS AND THE METHOD COMPRISING APPLYING THE THUS MODIFIED ADHESIVE AT A TOTAL SOLIDS LEVEL CORRESPONDING TO BETWEEN ABOUT 80% AND ABOUT 90% OF THE LEVEL NORMALLY REQUIRED IN THE CASE OF RESORCINOL-BASED RESIN ADHESIVES NOT CONTAINING VINYL ACETATE POLYMERS WHEREBY BOIL-PROOF AND WEATHER-RESISTANT BONDS ARE CREATED NOT SUBSTANTIALLY DIFFERENT FROM THE BONDS OBTAINABLE WITH THE UNMODIFIED RESORCINOL-BASED RESIN UNDER THE SAME CONDITIONS OF ASSEMBLY AND CURE.

United States Patent US. Cl. 156-332 5 Claims ABSTRACT OF THE DISCLOSUREThis invention relates to a method of gluing wood, wood articles and thelike with resorcinol-based resin adhesives, said adhesives comprising aresorcinol-based resin and a vinyl acetate polymer in an amount up toabout of the resorcinol-based resin solids and the method comprisingapplying the thus modified adhesive at a total solids levelcorresponding to between about 80% and about 90% of the level normallyrequired in the case of resorcinol-based resin adhesives not containingvinyl acetate polymers whereby boil-proof and weather-resistant bondsare created not substantially different from the bonds obtainable withthe unmodified resorcinol-based resin under the same conditions ofassembly and cure.

This application is a divisional application of Ser. No. 809,391 filedMar. 21, 1969 now Patent No. 3,600,353.

BACKGROUND OF THE INVENTION Resorcinol-based phenolic resins have foundspecial places for themselves in the adhesive industry because they canform bonds which are durable to boiling water and to outdoor exposure,and particularly because they are capable of setting or curing withoutthe addition of cellulose-degrading acid catalysts and at relatively lowtemperaturs, even at ambient temperatures. For these reasons,resorcinol-based resins are desirable, for example, in glues forlaminating structural timbers and in secondary adhesives used forassembling articles such as furniture which cannot be convenientlysubmitted to a high-temperature curing cycle. However, a greatdisadvantage attached to the use of resorcinol-based resins has beentheir high cost compared to the straight phenolic resins.

Attempts have been made to lower the cost by simply diluting the resinswith water or other solvents and applying lower amounts of solidsbetween the adherend surfaces, but these attempts have beenunsatisfactory since the viscosity of such dilutions is not sufiicientfor proper spreading. Lower solids resins can be made to desiredspreading viscosity by adjusting the proportion of formaldehyde, butsuch solutions are unsatisfactory because they have excessively slowspeeds of curing.

Attempts to incorporate other materials and still maintain the inherentadvantages of resorcinol-based resin have been unsuccessful. Manymaterials had to be eliminated because they were not suificientlycompatible with resorcinol-based resins. Other materials includingmethyl cellulose, polyvinyl alcohol, and highly carboxylated acryliccopolymers, although compatible, give mixtures with undesirableviscosity characteristics, the viscosity either being too high from thestart or changing rapidly during the period of pot life.

SUMMARY OF THE INVENTION Novel resorcinol-based resin adhesives have nowbeen found which maintain the level of desired properties of 3,681,168Patented Aug. 1, 1972 such adhesives, but at lower levels ofresorcinol-based resin solids. A method has also been found to adherecellulosic materials whereby the novel adhesives of the presentinvention are used at solids levels less than that used withresorcinol-based resin adhesives of the prior art. The net result ineach case and overall is a greatly reduced cost. In short, the instantinvention comprises an aqueous adhesive solution comprising aresorcinolbased resin and up to about 10% by weight, based on the resinsolids, of a vinyl acetate polymer and also comprises the method ofadhering cellulosic substrates including the steps of applying saidadhesive solution admixed with the usual hardener to at least oneadherend surface and curing the adhesive to form the substrates into aunitary assembly.

It has been surprisingly found that adhesives prepared according to thisinvention have substantially the same flat viscosity characteristics asthe parent resorcinol-based resins and that the adhesive bonds providedby the method of this invention even when cured at temperatures betweenabout 60 F. and about 120 F., have substantially the same strength andresistance to boiling water and weather exposure as are provided by theparent resorcinol-based resin alone even when total solids appliedbetween adherend surfaces is reduced to about 80% of the level requiredin the case of the resorcinol-based resin adhesives without the vinylacetate polymers.

DETAILED DESCRIPTION The two main constituents of the adhesives of theinstant invention are the resorcinol-based resin and the vinyl acetatepolymer.

As used herein the term resorcinol-based resin refers to anywater-soluble or water-dispersible resorcinolphenol-formaldehydecondensate of the novolak-type comprising suificient quantity ofresorcinol to confer the desired capability of curing at low temperatureand forming adhesive joints resistant to boiling and to weather. Whileit is possible to omit phenol entirely, it is known that up to of theresorcinol on a molar basis may be replaced by phenol without losingmost of the benefits of resorcinol. In general when phenol is used amole ratio of resorcinol to phenol equal to between 40/60 and 60/ 40 ispreferred for optimal balance of properties and cost.

With the present cost of materials, resorcinol and phenol themselves arepreferred, but products with comparable properties can be made with aportion, and in some instances all of the resorcinol replaced by otherpolyhydroxy benzenes such as catechol, hydroquinone, pyrogallol,hydroxyquinol, 1,2,4-trihydroxy benzene, phloroglucinol, and mixturesthereof. In like manner, phenol itself can be replaced by any of thecresols, xylenols and the like and their mixtures. Limited amounts ofvegetable tannins may also be included.

Methods for preparing resorcinol-based resins are wellknown in the artas illustrated, for example, in US. Patents 2,490,927 and 3,210,309 andare not part of the instant invention. Usually, resorcinol and phenolmay first be mixed and then reacted with formaldehyde. Alternatively,the resorcinol and phenol may be reacted with formaldehyde in separatestages. It is commonly accepted that similar products are obtained bymeans of these two methods, with end properties depending largely on themole ratio of resorcinol to phenol and on the pH of reaction. In anyevent, resorcinol-based resins made by any of the prior art proceduresare suitable for use in the adhesives of the present invention.

While formaldehyde is the preferred aldehyde for present economicreasons as well as technical reasons, comparable products can be madereplacing all or part of said formaldehyde with other aldehydes such asacetaldehyde,

3 paraformaldehyde, propyl aldehyde, any butyl aldehyde, furf-urylaldehyde, or mixtures thereof. The use of the term formaldehyde hereinis therefore to be understood as designating this generic class ofaldehydes.

Resorcinol-based resins are commonly cured by the addition of a hardenerand such hardeners are equally effective with the adhesives of thisinvention. They include various methylene compounds such as formaldehydeitself, certain formaldehyde compounds, paraformaldehyde,alpha-polyoxymethylene. When curing is to be carried out at elevatedtemperatures, it is possible to use hexamethyl' enetetramine. Thepreferred hardening agent is alpha-polyoxymethylene as it assists inproviding the desired curing at room temperature or above at a goodrate.

The vinyl acetate polymer to be used in carrying out this invention maybe selected from the group consisting of vinyl acetate homopolymers andcopolymers of vinyl acetate with at least one olefinically unsaturatedcoreactive monomer. Said polymers may be prepared by any of the knownmethods of polymerizing vinyl monomers including solution, suspensionand emulsion polymerization procedures. Latexes formed by emulsionpolymerization are preferred because of their relative ease ofincorporation into the resorcinol-based resin solution. Such latexes maybe synthesized with the assistance of any of the usual surfactants andof polymeric colloidal supporting agents. When surfactants are used itis preferred that these be anionic in nature in order to enhancecompatibility with the resin system. Usually employed in amounts ofbetween 0.1% to .3% based on the vinyl acetate, anionic surfactantswhich have proved particularly successful include the dioctyl ester ofsodium sulfosuccinic acid (e.g. Aerosol OT), sodium lauryl sulfate (e.g.Duponol WAQ), and the sodium salts of various alkyl aryl polyethersulfonates (such as Triton X-200).

While vinyl acetate homopolymers are preferred, the maintenance ofsubstantially the same properties in the adhesives of this invention maybe achieved by the use of copolymers of vinyl acetate with up to about20 Weight percent of a coreactive non-ionic monomer such as vinylpropionate, vinyl formate, ethyl acrylate, 2-ethylhexyl acrylate, butylacrylate, or mixtures thereof. It is also advantageous to polymerizewith up to about 5% by weight of a carboxylated coreactive monomer suchas acrylic acid, methacrylic acid, itaconic acid, maleic acid,methacrylic acid dimer, and mixtures thereof. Terpolymers of vinylaetate and said non-ionic and carboxylated coreactive monomers are alsosuitable.

The mixtures of the resorcinol-based resin and the vinyl acetate polymermay be made by the user at the time of applying the adhesive, just priorto the addition of the hardening agent. Alternatively, the mixture canbe effected at the place of manufacture, for it has surprisingly beenfound that mixtures can be made which are stable to storage for manymonths and which may therefore be shipped in a ready-to-use form.

Various fillers and miscellaneous materials that are commonly used inwood adhesives may be and suitably are incorporated in the adhesives ofthis invention in their usual proportions. Example of fillers thatillustrate such use are walnut or other nut shell flour, ground Douglasfir bark, wood flour, fine asbestos, or powdered clay. Fillers may beintroduced into the adhesive formulation directly or first admixed withthe hardener, or both. When paraformaldehyde or alpha-polyoxymethyleneare used as the hardener, admixture in dry form with a small quantity ofa finely divided filler such as Wood-flour, facilitates the dispersionof the hardener into the resin-vinyl acetate adhesive solution.

As to proportions, not more than about by weight of vinyl acetatepolymer solids, based on the weight of resorcinol-based resin solids,should be used since the desired characteristics of certain of theresorcinol-based resin adhesives may be adversely affected. The lowerlimit used is about 1%, with lower levels effective, but not resultingin the cost savings desired. The surprising feature of this invention isthat with the addition of thevinyl acetate polymer the adhesive can beused at solids levels (resorcinol-based resin solids plus vinyl acetatepolymer solids) less than required using adhesives containing onlyresorcinol-based resin solids. Illustrated in Example 2, hereinafter isthe fact that an adhesive of the instant invention gives the sameadhesive properties as a resorcinolbased resin adhesive of the' priorart at the same glue spread even though the solids content thereof is85.4% of that of the prior art adhesive. Broadly, adhesives of thepresent invention can be used at solids levels as low as of thatordinarily used in the existing resorcinolbased resin adhesives.

The adhesive of this invention may be applied in any of the wayspracticed in the application of resorcinol-based adhesives includingbrushing, spreading with glue spreader, spraying or using a glue gun. Ineach of these methods, the amount of applied adhesive is controlled bymaking adjustments which depend on changes that may occur in theviscosity of the adhesive during storage or changes which may occur inthe blend with hardener during its pot life. The preferredresorcinol-based resins used in preparing the adhesives of thisinvention have the advantage of possessing relatively the same viscosityregardless of whether the blend with hardener is made directly aftermanufacture of the resin or months later. Also, the blends with hardenerdo not change rapidly during the time of pot life. Formulations whichhave such relatively constant viscosity behavior are said to have flatviscosity characteristics. The adhesives of this invention have theunexpected advantage that their viscosity behavior is very similar tothat of the parent resorcinol-based adhesives and thus also have thedesirable flat viscosity characteristics.

Curing of the adhesives of this invention may be carried out underconditions similar to those used in the case of the prior-artresorcinol-based resin adhesives, including curing at ambienttemperature.

A factor of particular significance in processing is the open assemblytime by which is meant the time between the application of the adhesiveto the substrate and the assembly of the treated surfaces. It is anadvantage to have the open assembly time relatively long so that therequired operations may be carried out without the adhesive beingprematurely hardened or redistributed in a manner so that the adhesivebond would be defective. The adhesives of the instant invention have theadvantage that they permit assembly times up to thirty minutes or longerin the open state at ambient temperature.

The invention will be more clearly illustrated in the following examplesof the practice of it wherein the properties of the cellulosic productshave been tested in accordance with the applicable government andidnustry procedures.

Example 1 A still with reflux condenser and stirrer was charged with1645 parts by weight of an 87% aqueous solution of phenol, 1250 parts ofa 37% aqueous solution of formaldehyde containing 0.7% methanol, parts50% aqueous causic soda solution and 310 parts of water. The mixture washeated to 90 0., held at that temperature for 30 minutes and thencooled. Next 1200 parts of resorcinol were added and the mixture wasreheated and maintained at reflux for several hours until a Gardnerviscosity of V was reached. The product was then recooled and there wasadded a mixture of 200 parts water with 300 parts of a denatured alcoholof the composition 94% by weight 95% ethanol, 5.3% ethyl acetate and .85aviation gasoline. The resulting resorcinol-based resin solution hadsolids content of 54.8% and a viscosity of 364 cps. at 21 C.

A solid hardener was prepared by dry-blending 50 parts ofalpha-polyoxymethylene with 28 parts walnut shell flour and 22 partsDouglas fir flour.

A 79.0 part portion of the resorcinol-based resin solution was extendedwith 2.3 parts of a polyvinyl acetate homopolymer emulsion containing55% solids, 14.1 parts water, 4.6 parts of a denatured alcohol (Solox),and 16 parts of above-described solid hardener. Small Douglas fir beamswere glued by applying this composition at a spread of 80 lbs. per 100sq. ft. at 70 F. Tests were made with varying times allowed between thespreading and the assembly of the beams. In one case the beams wereassembled immediately after application of the glue; in other cases theassembly was carried out after 10 minutes and 60 minutes respectivelyafter application of the glue. Each assembly was clamped at a pressureof 125 p.s.i. and kept in clamped condition overnight at a temperatureof 7075 F. After removal of the clamps the assemblies were aged forseveral days at room temperature and then tested. In all cases shearstrength averaged 310 p.s.i. and wood failure was 83%. Delaminationtests (AITC-llO) showed less than 1% delamination. These results weretypical of those obtainable with the normal application, at the samespread of 80 lbs. per 1000 sq. ft., of the nonextended resorcinol-basedparent resin and also are better than the wet-use requirements ofspecification CS-253-63 that shear strength should be about 1050 p.s.i.with at least 70% wood failure and that delaminations should be lessthan 10%.

Example 2 A sample of 55 solids polyvinyl acetate homopolymer emulsionwas cut to calculated solids of 20% with a mixture of equal partsacetone and denatured alcohol, resulting in a hazy solution.

A portion of the resorcinol-based resin of Example 1 was extended bymixing 69.5 parts of said resin with 17.4 parts of this 20% polyvinylacetate solution and 13.1 parts of denatured alcohol, corresponding to 9parts of poly-vinyl acetate per 100 parts of resorcinol-based resinsolids. To this extended formulation there was added 14.0 parts of thedry-blended hardener of Example 1 and the mixture was used to glue firbeams according to the procedure of Example 1. The obtained shearstrengths were 1295 p.s.i. with 90% wood failure and a delaminationvalue of 1.4%, These values again exceed the requirements ofspecification CS-253-63 for wet-use.

Example 3 A vessel equipped with a mixer was charged with 83.5 parts ofa resorcinol-phenol resin of commerce, Cascophen RS240MD, containing 53%solids, 5.2 parts of a denatured alcohol (Solox) and a dilution of 2.6parts of a polyvinyl acetate copolymer emulsion, containing 55% solids,with 5.2 parts of water. When the mixture had become homogeneous therewas further added 3.5 parts more of water. Prior to using thiscomposition as a glue there was added a hardener of commerce based uponalpha-polyoxymethylene .(FM-124-D). This adhesive composition containedabout 3.2 parts of polyvinyl acetate solids per 100 partsresorcinol-based solids. This modified resin adhesive was used to glueplywood of three-eights inch thickness to the two-inch surface of apiece of 2 x 4 lumber in a hot press with platen at temperature of 430The total solids applied represented an economy at current prices ofover 20% in cost of adhesive materials compared to the normal use of thenon-extended resorcinol-based resin of commerce. The curing cyclerequired to obtain a bond sufficiently strong for chiseling afterremoval from the hot press and cooling was 5 minutes.

Example 4 Using the procedure of Example 3, an adhesive was prepared byblending 80 parts by weight of a commercially available resorcinol-basedresin, Cascophen LT-71 containing 53% solids with 3.0 parts of apolyvinyl acetate homopolymer emulsion comprising about 5% polyvinylalcohol based on the weight of polyvinyl acetate, and 17 parts of water.Great care was taken to homogenize this blend thoroughly. An aliquot ofthis blended adhesive was formulated with a commercial hardener FM-282based on alpha-polyoxymethylene in the proportions of 16 parts hardenerper 100 parts by weight of the adhesive. Small beams of oak were gluedwith this formulation containing hardener, using lumber conditioned at70 F. Spreads were made at lbs. per 1000 sq. ft. and at three differentassembly times including immediate (closed) assembly and with openassembly times of 15 minutes and 75 minutes respectively. The assemblieswere cured overnight at 70-75 F. and 125 p.s.i. After aging at roomtemperature for several days, dry shear strength averaged 3050 lbs.p.s.i. with no significant difference among samples corresponding todifferent assembly times. The weathering test showed 3.9% delamination.These results met the requirements of specification MIL-A-397B whichstipulates that the dry shear strength should be at least 1800 lbs. persquare inch, the wood failure greater than 75% and the delamination lessthan 8%.

Example 5 The formulation with hardener described in Example 4 was usedto glue maplebeams under the same conditions as set forth in Example 4.The dry shear strength obtained was 3180 p.s.i. as compared to 2800required by specification MIL-A-397B. Furthermore, the tests showed 68%wood failure whereas said specifications have no wood failurerequirement.

Example 6 Birch plywood was laminated using the adhesive and procedureof Example 4. Dry shear strength obtained was 410 p.s.i. as comparedwith the 400 p.s.i. requirement of specification MILA-397B. The woodfailure was 47%. After a 48-hour standard water soak the shear strengthwas 550 p.s.i. with 98% wood failure. After a standard 3-hour boil theshear strength was 520 p.s.i. with 99% wood failure.

Example 7 Small beams of Douglas fir were glued and tested according tothe procedures of Example 4, a dry shear strength equal to 1710 p.s.i.with 97% wood failure was obtained. As was the case in all previousexamples the test results did not vary significantly with assembly time,up to 75 minutes.

Example 8 The modified adhesive of Example 4 made by blending CascophenLT-7l, polyvinyl acetate latex and water was found to be stable at roomtemperature storage for several months. Gluing tests made with thestored adhesive according to the procedures used in the precedingexamples give substantially the same results as initially.

Example 9 In a manner similar to that of Example 3, a blend was made byextending 77.5 parts of resorcinol-based commercial resin PenacoliteG-4422 with 4.7 parts of a 55% solids latex of a vinyl acetate copolymercomprising .2% acrylic acid based on vinyl acetate, 9.12 parts of waterand 8.4 parts of Solox denatured alcohol. Small Douglas fir beams wereglued with mixes incorporating 16 parts by weight of G-4400 commercialhardener per 100 parts of the blend of this example. Tests were carriedout by procedures similar to those of the preceding examples usingrepresentative conditions of glue spread and temperature and time oflumber assembly. The results summarized in the accompanying tableindicate that the beams were all well glued exceeding the requirementsof specification CS-253-6 3. In every case the delamination 'was lessthan 1% in weathering test AITC-llO involving alternate cycles of vacuumand pressure. The properties throughout corresponded satisfactorily tothose obtained with equivalent spreads of straight Penacolite G-4422 butamounted to a saving of at least 20% in coat of raw materials at thecurrent price levels.

TABLE L-ASSEMBLY Lbs. glue Lumber Minutes Shear Percent spread pertemperature, strength, wood 1,000 sq. ft. degrees F. Open Closed p.s.failure Example The procedure of Example 1 is followed except that thepolyvinyl acetate homopolymer emulsion is replaced separately and inturn by emulsion copolymers of vinyl acetate with respectively 5, 15,and parts of respectively vinyl propionate, vinyl formate, ethylacrylate, 2-ethylhexyl acrylate, butyl acrylate, a 1:1 mixture of vinylpiopionate and ethyl acrylate and a 1:3 mixture of vinyl formate andbutyl acrylate.

In all cases bond strengths are good and wood failure is high.

Example 11 The procedure of Example 2 is repeated except that the 20%polyvinyl acetate solution is made by polymerizing vinyl acetate monomerin methanol solution using 2-azobis isobutyro nitrile as initiator and,upon completion of polymerization, adjusting the solids to 20% byappropriate addition of methanol. The gluing tests result in similar dryshear strength, high percentage wood failure and low delamination.

Example 12 The procedure of Example 2 is repeated except that the 20%polyvinyl acetate solution is made dissolving in methanol theappropriate amount of a vinyl acetate suspension polymer made bypolymerizing vinyl acetate in a suspension comprising .0124 part byweight of polyvinyl alcohol based on vinyl acetate in the usual manner.The glue tests again result in high dry shear strength, high percentagewood failure and low delamination.

The adhesives of this invention are described herein as particularlyuseful in the laminating of lumber such as structural beams and in thesecondary gluing of wood. It is to be understood, however, that they areexcellent bonding agents for other cellulosic material such as paper,laminated plastics, particle board, rockwool and the like.

It will be understood that it is intended to cover all changes andmodifications of the examples of the invention herein chosen for thepurpose of illustration which do not constitute departures from thespirit and scope of the invention.

What is claimed is:

1. A method of adhering at least two cellulosic substrates whichcomprises applying to the surface of at least one of said substrates anadhesive composition, bringing the substrates into contact, and curingthe adhesive composition to form the substrates into a unitary assembly,the adhesive composition comprising a solution of aphenol-resorcinol-formaldehyde novolac resin containing sufiicientquantity of resorcinol to confer the desired cap ability of curing atlow temperature and forming adhesive joints resistant to boiling and toweather, and a vinyl acetate polymer containing at least of vinylacetate, said polymer being present in an amount, on a solids basis,from about 1% up to about 10% by weight, based on the resin solids.

2. The method of claim 1, wherein the total amount of adhesive solidsapplied is approximately 15-30 pounds per 1000 square feet of surfaceand wherein the curing is carried out at a temperature below about 120F.

3. The method of claim 1, wherein the resin contains from about 25 to 75parts by weight of phenol for each parts by weight of resorcinol andphenol and from about 0.5 to about 0.9 mol of formaldehyde per mol ofphenol and resorcinol, said vinyl acetate polymer is a copolymer ofvinyl acetate with at least one olefinically unsaturated reaetivemonomer, said adhesive composition including alphapolyoxymethylenehardener and a particu late filler, said hardener and said particulatefiller each being present in the proportion of from about 10 to about 40parts by weight for each 100 parts by weight of said resin and polymersolids combined.

4. The method of claim 3, wherein the total amount of adhesive solidsapplied is approximately 15-30 pounds per 1000 square feet of surfaceand wherein the curing is carried out at a temperature below about F.

5. The method of claim 4, wherein curing takes place at ambienttemperature.

References Cited UNITED STATES PATENTS 2,699,417 '1/1955 Repsher et al156-335 X 3,210,309 10/1965 Baker et a1. 260-293 2,609,352 9/1952Kualnes l56-335 UX 3,041,301 6/1962 Armour 156332 X 3,471,443 10/1969Bornstein 156-335 X CARL D. QUARFO'RTH, Primary Examiner E. A. MILLER,Assistant Examiner US. Cl. X.R. 156335

